Intergradation among Latin American maize based on an analysis of chromosome knob frequencies

Published information on chromosome knobs found at 21 knob-forming positions and on abnormal 10 and B chromosomes in maize, Zea mays L., was used to place maize populations within a multidimensional space based on frequencies. From this space, similarities among populations were determined using a measure of gentic diversity based on a modified Cartesian distance. Populations were portrayed in 2 (or 3) dimensions based on these distances. The objective was to investigate patterns of "migration" that had occurred among indigenous populations of maize from Latin America. Widely dispersed collections classified as Tuxpeño had similar knob constitutions. Collections from Guatemala reflected continuous migration among adjacent areas with increased isolation (or association of knob types) with increased altitude of collection. Maize from southeastern Guatemala and their southeastern neighbours were similar. The high elevation collections from Guatemala and Mexico were surprisingly similar. The data reflected three distinct phenomena: long-term intergradation of maize germplasm among adjacent areas (as would result from pollen drift between closely cultivated areas or from seed exchange among neighbors), major, relatively recent shifts in gene flow (as had occurred with Tuxpeño's widespread distribution in Mexico), and precolonial dispersions (as between maize populations from the high elevations in Guatemala and Mexico).